I would suggest that you invest in a 77mm filter and a 77 to 72 step down ring. Many of canon's L lenses are 77mm, so the filter can be used on them or with a step down kit, on almost any lens.

It's worth knowing about step-up rings, as a way to use one very expensive filter on two lenses, but they are best avoided because the larger filter can prevent the use of a lens hood. Using a lens hood is always important, but even more so when using a filter because of the increased chance of flare.

Also there is no point in paying top dollar for a brass rim if you then have to use some cheap metal step up ring - and high quality brass step up rings are surprisingly expensive. And that's on top of the higher cost of a 77mm vs 72 mm filter, to fit a hypothetical future lens which the OP may never own.

I actually have four or five step up rings and one step down ring, so I do appreciate their usefulness - but only to cover occasional requirements, not for everyday use.